1. Field of the Invention
The field of the invention is that of optical devices making it possible to measure the orientation of an object in space without contact. There are various possible fields of application, but the main application is that of detecting the posture of the helmet of an aircraft pilot, thus making it possible to project an image into his visor in exact superposition on the exterior landscape or to slave various systems of the aircraft under his view. The precision sought in such systems is of the order of one milliradian.
2. Description of the Prior Art
There are various optical techniques for measuring the orientation of a helmet. Generally, conspicuous elements are installed on the helmet, and these are located by an optical emission and reception system. The positions of the images of these conspicuous elements make it possible to determine the position and the orientation of the helmet by calculation.
To this end, retroreflective corner cubes or retroreflectors may be used. It is sufficient to arrange the optical emission and reception components on the same axis. Owing to their nature, these systems with retroreflectors have low sensitivity to sunlight.
As an exemplary embodiment, they may be combined with a fixed optoelectronic device comprising a point source associated with an assembly comprising one or two matrix sensors without an optical objective. In this arrangement, the reflector is equipped with a mask applied onto its entry face. This mask comprises a transparent central part and an opaque peripheral part. The contour of the mask is in the shape of a parallelogram, thus embodying the orientation of two fixed directions of the helmet. The orientation of the helmet is calculated by analysing the shapes of the contour which are projected onto the sensor or sensors. The analysis relates to the transitions between the light and dark zones of the reflection received by the sensor.
A first fixed optoelectronic device of this type is represented in FIG. 1. FIG. 1 is simplified view in section in a plane (X, Z). The device comprises a point source S, a first splitter plate m making it possible to separate the emission path and the reception path, and a second neutral splitter plate m making it possible to send the light reflected by the corner cube onto two photosensitive sensors CCD1 and CCD2, which are arranged at different positions. Thus, the images of the source S which are given by the plates m and m′ are the points S′1 and S′2 on the sensors CCD1 and CCD2. A variant of this first device is represented in FIG. 2. In this variant, the second splitter plate m′ is replaced with a set of two plates mr and m″. The first plate mr is dichroic, it reflects a first wavelength range and transmits the rest of the spectrum. The single sensor CCD therefore receives two offset shapes of the projected contour, the first in a first wavelength range and the second in the rest of the spectrum. Thus, the images of the source S which are given by the plates m and m′ are the points Sr and Sv.
These fixed optoelectronic devices have some drawbacks, which are detailed below:                The fixed optoelectronic device comprises a certain number of optical elements and two matrix sensors. It is therefore bulky and requires double image acquisition electronics;        The light source emits in an angular aperture wide enough to cover all the possible positions of the reflector. A very large part of the light flux is therefore permanently lost, since it does not reach the reflector. This flux part is furthermore radiated into the surrounding space inside and outside the cockpit thus impairing the stealth of the device;        The unknown position of the vertex of the corner cube reflector is determined either by analysing the image projected onto each of the two sensors or by analysing two coloured images. The uncertainty relating to the position of the vertex of the reflector, on the one hand, and the uncertainty relating to its orientation, on the other hand, are greater when the two sensors or the two images are closer to one another in orientation and/or position.        